Gaussian Process registration integrated into statismo

This pull request integrates methods for performing non-rigid registration using statismo. 

The work is based on the paper:
"A unified approach to shape model fitting and non-rigid registration"
Marcel Lüthi, Christoph Jud and Thomas Vetter
(http://gravis.cs.unibas.ch/publications/2013/MLMI-Luethi_etal-2013.pdf)